Sintering Neck Growth Research Articles

Abnormal shrinkage behavior and sintering mechanism of alumina nanoparticles

AbstractHigh temperature and ultrafast heating rate, the characteristics of ultrafast high‐temperature sintering (UHS) process, make it difficult to capture the evolution of sintering behavior and explore sintering mechanism of alumina ceramics. In this study, molecular dynamic simulations are adopted to describe the sintering behaviors of equal‐sized and unequal‐sized alumina nanoparticles, particularly about shrinkage behaviors, microstructure evolution and atomic migration. Results show that high temperature and ultrafast heating rate can yield two different abnormal shrinkage behaviors, accelerating the polymerization of the nanoparticles. The main mechanism for this phenomenon is that high temperature causes the amorphization of crystal structure of nanoparticles, and ultrafast heating rates retain larger sintering driving forces at high temperatures, leading to the reduction of diffusion activation entropy and migration barriers. In particular, the appearance of viscous flow further enhances the shrinkage of the nanoparticles. In addition, surface diffusion is found to dominate the growth of sintering neck during this period. These findings are expected to deepen the understanding of ultrafast high‐temperature sintering mechanism.

Coalescence behavior of size-selected gold and tantalum nanoclusters under electron beam irradiation: insights into nano-welding mechanisms.

The emerging technique of nano-welding (NW) via precisely regulating the fusion of nanoclusters (NCs) in nanotechnology has attracted significant attention for its innovative approach. Employing the gas-phase condensation cluster source with a lateral time-of-flight (TOF) mass-selector, size-selected gold (Au), and tantalum (Ta) NCs were prepared. This study explores the coalescence behavior of size-selected Au and Ta NCs under electron beam irradiation, aiming to investigate the related mechanism governing the welding process. Intrinsically driven by the reduction of excess surface energy, electron beam induces atomic thermal migration, fostering sintering neck growth at cluster interfaces. During this process, atomic diffusion and recrystallization enable NCs to alter shape while retaining stable facet planes. Aberration-corrected scanning transmission electron microscopy (AC-STEM) showcases the formation of single or polycrystalline sintered clusters, during which some lattice distortions can be eliminated. Interestingly, oxidized Ta clusters experience knock-on damage caused by elastic scattering of electron beams, partially deoxidizing them. Additionally, electron-phonon inelastic scattering transforms oxidized Ta clusters from amorphous to crystalline structures. Moreover, the quantum size effect and surface effect of NCs facilitate the surpassing of miscibility limits during Au-Ta heterogeneous welding processes. This investigation bridges the gap between fundamental research on cluster materials and their practical applications.

An insight into sintering mechanisms of silicon carbide nanoparticles with additives using MD simulation

The effect of sintering additives on the sintering of SiC nanoparticles is studied by MD simulation. A new potential function is developed based on the lattice size theory. The Al-Si-C element system, Ti-Si-C element system, and V-Si-C element system are simulated and compared. MD simulation can be used to select the sintering additives of SiC ceramics and Al is better by atomic diffusion analysis. The nanoparticle sintering evolution processes with/without aluminum are studied in detail and compared with the typical sintering experimental results. The sintering mechanisms of nanoparticles are quantitatively analyzed by sintering neck growth, energy evolution, and atomic diffusion. The results show that it is beneficial for selecting an appropriate temperature to obtain the highest sintering promotion effect with additives. Kinds of sintering mechanisms are summarized and in good agreement with the traditional sintering theory. The results here are helpful to understand the sintering underlying mechanism of SiC ceramics.

Numerical investigation into pressure-assisted sintering using fully coupled mechano-diffusional phase-field model

To investigate the effect of contact stress between particles on the evolution of microstructure in pressure-assisted sintering, an improved fully coupled mechano-diffusional phase-field model (PFM) was presented, in which the design of the interpolation method for elastic modulus ensured that the normalised contact stress distribution was approximated to that in conventional contact problems. The immediate advantage was that the consequent strain energy in the contact and neck area could be directly characterised without additional approximate treatment.The simulation results showed that the sintering neck evolved under the joint action of the mechanical mechanism of elastic contact and the mass diffusion mechanism for pressure-assisted sintering. Elastic contact played a dominant role before significant neck growth in the early stage of sintering. As the neck grew, mass diffusion became the dominant factor, but the normalised contact stress distribution in the contact area always satisfied that of the elastic contact problem. Owing to the additional contribution of strain energy, the greater the pressure, the faster the sintering neck growth. Further study showed that the diffusion-induced stress hindered the growth of the sintering neck according to the presented fully coupled model presented, and that the strain energy stored within the contact area will accelerate the sintering process.

Sintering mechanism of copper nanoparticle sphere-plate of crystal misalignment: A study by molecular dynamics simulations

Sintering mechanisms of copper nanoparticles on copper plates are investigated based on molecular dynamics (MD) simulation. In this paper, the roles of tilt grain boundaries (GBs) and Σ5 twisted GBs on sintering kinetics are considered. Regardless of the diameter of the nanoparticle sphere, it is observed that stacking faults and dislocations are formed near the GBs by crystal analysis at the early stage of sintering and throughout the entire sintering process. The stacking faults are overflowed to the surface during the constant temperature stage. The tilt GBs disappears rapidly at the initial stage of sintering, while the Σ5 GBs is instantly eliminated in the middle temperature region of sintering by the rigid rotation of the nanoparticle sphere. Moreover, besides the contribution of atom diffusion in sintering neck growth, misalignment between the two particles brings about enhanced GBs diffusion. These observations provide insights into the tailored high-angle GBs applicable to nano-scale sphere-plate samples.

Effect of sintering atmosphere on the microstructure and properties of nano-WC modified copper-iron-based oil-containing bearing

Nano-WC modified copper-iron-based oil bearing was prepared by powder metallurgy. The effect of sintering atmosphere on the microstructure, crushing strength, hardness and oil content of the oil bearing were studied. The results show that under the sintering condition of 865 °C×30min, when the sintering atmosphere is 100%N2, the sintering process stays in the early stage of sintering neck growth, there are a large number of irregular pores in the microstructure. The sintering densification is insufficient, and the sample is in the state of undersintering. When the sintering atmosphere is 100%H2, the dezincification of matrix phase α(Cu) is serious, and there are many long strip microcracks between metal particles. The effective connection between α(Cu) and Fe phase is cut off, so the density of sample is very low. When the sintering atmosphere is 75%H2+25%N2, α(Cu) forms a united whole through sufficient sintering diffusion bonding, and encapsulates the Fe-rich phase. The sintering densification is very sufficient, the pores in the microstructure tend to be circular. The sintered samples have good comprehensive properties, the density and oil content are 6.60 g/cm3 and 18.21vol.%, the crushing strength and hardness are 235.81 MPa and 42.41 HB, respectively, showing the best sintering state.

Process Optimization Under Uncertainty for Improving the Bond Quality of Polymer Filaments in Fused Filament Fabrication

Abstract This paper develops a computational framework to optimize the process parameters such that the bond quality between extruded polymer filaments is maximized in fused filament fabrication (FFF). A transient heat transfer analysis providing an estimate of the temperature profile of the filaments is coupled with a sintering neck growth model to assess the bond quality that occurs at the interfaces between adjacent filaments. Predicting the variability in the FFF process is essential for achieving proactive quality control of the manufactured part; however, the models used to predict the variability are affected by assumptions and approximations. This paper systematically quantifies the uncertainty in the bond quality model prediction due to various sources of uncertainty, both aleatory and epistemic, and includes the uncertainty and the model discrepancy in the process parameter optimization. Variance-based sensitivity analysis based on Sobol’ indices is used to quantify the relative contributions of the different uncertainty sources to the uncertainty in the bond quality. A Gaussian process (GP) surrogate model is constructed to compute and include the model discrepancy within the optimization. Physical experiments are conducted for calibration and validation of the physics model and also for validation of the optimum solution. The results show that the proposed formulation for process parameter optimization under uncertainty results in high bond quality between adjoining filaments of the FFF product.

Sintering neck growth mechanism of Fe nanoparticles: A molecular dynamics simulation

In this paper, the sintering neck growth mechanism of Fe nanoparticles is studied by molecular dynamics method. The accuracy of the EAM potential function is verified. The relationship between the sintering temperature of the nanoparticles and the particle size has been investigated. The sintering temperature of the particles is linearly positively correlated with the size. The whole sintering process can be divided into three stages: the initial sintering, the stable growth and the sudden increase. The atoms are divided into surface atoms and internal atoms depending on the recognition method. By calculating the mean square displacement of the particles and observing the vector displacement of the atoms, the growth mechanisms of the sintering neck at different stages have been analyzed. Similarly, the evolution of crystal structure during sintering is captured by structure analysis. These simulation results firstly reveal the rule of atomic migration and the growth mechanism of the sintering neck.

The role of heating and cooling in viscous sintering of pairs of spheres and pairs of cylinders

PurposeThis study aims to develop mathematical models for the determination of the effects of heating or cooling on neck growth in Selective Laser Sintering (SLS) and Fused Filament Fabrication (FFF). Two particle shapes are studied: spherical and cylindrical.Design/methodology/approachThe time required for the coalescence (sintering) process is determined by balancing the work of surface tension forces and viscous dissipation. Heating and cooling effects are studied by incorporating temperature dependence of viscosity in an exponential form. Heating by a laser, convective and/or radiative heat transfer is assumed. It is also assumed that there are no temperature gradients within the coalescing molten polymers (lumped parameter heat transfer analysis).FindingsThe models predict faster sintering with heating and slower with cooling, as expected because of the effect of temperature on viscosity. For the isothermal case of pairs of cylinders, the present model predicts significantly longer time for completion of sintering than a previously developed and frequently cited model by Hopper.Originality/valueAn isothermal sintering model for two spheres was reworked for two long cylinders, and for the first time it has been compared to other models available in the literature. The mathematical models are capable of predicting neck growth under non-isothermal conditions for both spheres and cylinders. They are useful in assessment of bonding in selective laser sintering and fused deposition fabrication.

Study on the microwave sintering characteristics of spherical tin-silver alloy powder

In this study, the sintering behavior of a tin–silver alloy powder (Sn–3% Ag) is investigated during microwave melting, and a two-sphere model is used to study the main diffusion mechanism of sintering. The effect of microwave on the sintering process, microstructure and composition distribution of alloy was studied. The sintering neck growth of the tin-silver alloy powder suggests that the diffusion process of alloy components has been realized through the viscous flow mechanism during the sintering stage before melting under microwave irradiation. The rapidly heating and melting of tin-silver alloy powder can be realized by microwave heating, and the heating rate can reaches to 153.4 °C·min−1 at microwave power of 1.2 kW. In addition, microwave also showed a good heating effect on the metal melt. Microwave can promote the homogenization of alloy microstructural, and the precipitated phase of the alloy is refined and the element distribution is more uniform under microwave irradiation compared to the traditional process. These technology provides a new way for the preparation and performance improvement of alloy materials.

Effect of oxygen contents on predominant sintering mechanism during initial stage of pure titanium powder

This study investigates the sintering neck growth behavior of pure titanium powder with different oxygen contents. The growth rate of the sintering neck gradually decreased as the oxygen content increased from 0.06 wt% to 0.50 wt%. At the same time, it was observed that the oxygen in the sintering neck spread to the low oxygen region as the size of the neck increased. A theoretical analysis indicated that surface diffusion is the dominant diffusion mechanism. However, under the hindrance of oxygen in the matrix, both surface diffusion and grain boundary diffusion slow down. Calculating the increase in the self-diffusion activation energy of Ti atoms indicated that oxygen was more resistant to surface diffusion. Predictably, grain boundary diffusion will become the dominant diffusion mechanism in the initial stage of sintering when the oxygen content exceeds 2.628 wt%.

PREPARATION OF Cu-SiO2/PA12 COMPOSITE POWDERS BY ELECTROLESS PLATING FOR USE IN SLS PROCESSING

In this work, SiO2-encapsulated copper particles/PA12 (Cu-SiO2/PA12) composite powders were prepared by electroless composite plating, and the laser sintering behavior was investigated. Results showed that Cu, Cu2O, CuO, and SiO2 (Cu-SiO2) composite particles were plated on the surface of KH550-modified PA12 powders. The Cu-SiO2 particles existed independently on PA12 surface, and the size was around 200 nm. The melting temperature and crystallization temperature of Cu-SiO2/PA12 composite powders were 183∘C and 150∘C. The results indicate that the selective laser sintering (SLS) process involved the contact of Cu-SiO2/PA12 powders, the formation of sintering neck, the growth of sintering neck, and the formation of fused solid. The Cu-SiO2 composite particles uniformly dispersed in the part due to surface tension, and the contact interface was good due to their similar polarity. The Cu-SiO2/PA12 SLS parts had excellent dimensional precision. The tensile strength of the 15[Formula: see text]W-sintered Cu-SiO2/PA12 specimen was 48[Formula: see text]MPa.

Modelling the Sintering Neck Growth Process of Metal Fibers under the Surface Diffusion Mechanism Using the Lattice Boltzmann Method

In this paper, the sintering neck growth process of metal fibers under the surface diffusion mechanism is simulated by using the Lattice Boltzmann method (LBM). The surface diffusion model is developed considering the geometrical characteristic of metal fibers. Then, the LBM scheme is constructed for solving the developed surface diffusion model. The sintering neck growth process of two metal fibers with different fiber angles is simulated by LBM. The simulated morphologies of sintering metal fibers well agree with ones obtained by experiments. Moreover, the numerical simulation results show that the sintering neck radius of two metal fibers is increased with the increase of fiber angle, which implies that the initial geometrical characteristic plays an important role in the sintering neck formation and growth of metal fibers.

Densification and microstructure evolution of W-TiC-Y2O3 during spark plasma sintering

Spark plasma sintering (SPS) is one of the methods used to achieve the low-temperature densification of refractory metal materials. In this study, powder prepared through a wet chemical method was consolidated via SPS at 1100 °C, 1200 °C, 1350 °C, 1600 °C, and 1800 °C to obtain a high-performance W-TiC-Y2O3 composite material. Densification was studied by analyzing the densification curve and changes in the microstructure of the samples. This process could be divided into three stages: the bonding stage, the sintering neck growth stage, and the shrinkage and spherification stage of closed pores. Surface diffusion and grain boundary diffusion played different roles in densification. The density, grain size, and Vickers hardness of the tungsten material increased significantly as temperature increased. This study evaluated the sintering process and provided a basis for obtaining high-performance tungsten materials through SPS.

Preparation of nickel/PA12 composite particles by defect-induced electroless plating for use in SLS processing

In this work, Ni particles/PA12 powders (Ni/PA12) and graphite oxide (GO)-encapsulated Ni particles/PA12 powders (GO-Ni/PA12) composite powders were prepared by defect-included electroless plating technique, and its laser sintered behaviour was investigated. Results showed that a lot of defects could formed on the surface of CH3COOH etched PA12 powders. The defects would induce Ni and GO-Ni particles independently plated on the PA12 surface. Adding GO in the plating solution would facilitate the deposition of Ni particles, GO, and NiO on the PA 12 surface, but inhibit the growth and the crystallinity of the Ni particles. The SLS process involved the contact of PA12 powders, the formation of sintering neck, the growth of sintering neck and the formation of fused solid. Sintering process could facilitate the re-arrangement of Ni particles due to surface tension and the growth of sintering neck. The Ni particles had well wettability, and the interfaces between Ni particles and PA 12 were contacted soundly. The tensile strength and bending strength of the 10 W-sintered Ni/PA12 specimen were 50 MPa and 60 MPa. But SLS process caused the serious aggregation of GO-Ni particles due to higher concentration, activity and surface area of GO-Ni particles.

Discussion on the Local Magnetic Force between Reversely Magnetized Micro Metal Particles in the Microwave Sintering Process

Synchrotron radiation computed tomography was applied to investigate Cu–Fe mixture microwave sintering in situ and to examine the magnetic force between reversely magnetized micro-metal particles in microwave sintering. Results revealed that the growth rate of the sintering necks between Cu–Fe particles and Cu–Cu particles around the iron particles distributed in a vertical direction was faster than that of the sintering necks in the horizontal direction. These phenomena were consistent with the possible influence caused by the magnetic force between metal particles, as shown in our simple particle model. The kinetic curves of sintering neck growth along the vertical and horizontal directions quantitatively revealed the difference in growth rates. The contributing factors of magnetic force in microwave sintering were subsequently discussed. The volume of iron particles was proportional to the influence of magnetic force, and their shape elicited a remarkable influence based on demagnetization effects. This study provided a useful basis for microwave sintering mechanisms and anisotropic material preparation.

Coalescence of Cu contacted nanoparticles with different heating rates: A molecular dynamics study

The coalescence, the initial stage of sintering, of two contacted Cu nanoparticles is investigated under different heating rates of 700, 350 and 233 K/ns. The nanoparticles coalesced rapidly at the initial stage when the temperature of the system is low. Then, the nanoparticles collided softly in an equilibrium period. After the system was increased to a high temperature, the shrinkage ratio, gyration radius and atoms’ diffusion started to change dramatically. The lower heating rate can result in smaller shrinkage ratio, larger gyration radius and diffusion of atoms. However, the growth of sintering neck is hardly influenced by the heating rate. The results provide a theoretical guidance for the fundamental understanding and potential application regarding nanoparticle sintering.

In situ investigation of Al-Ti mixed metal system microwave sintering by synchrotron radiation computed tomography

In this paper, in-situ investigation of Al-Ti mixed system's microwave sintering was carried out on the basis of synchrotron radiation tomography technique. Quantitative parameters including porosity and sintering neck growth were extracted from experimental images. Al-Ti mixed sample had a faster densification and sintering neck growth rate by comparing with the microwave sintering of pure aluminum. The possible mechanisms which accelerated the sintering process of Al-Ti mixed system were discussed. From the aspect of thermal effect, owing to the heterogeneous interface introduced by titanium particles, polarization loss would happen at the interface layer. Both polarization loss and eddy current loss contributed to the promotion of sintering. And from the aspect of non-thermal effect, there would be a gradient of microwave field at the heterogeneous interface resulted from the different microwave skin depth of intermetallic compounds and pure metal. Mass diffusion was supposed to be accelerated due to the electromagnetic field gradient.

In situ Investigation of Titanium Powder Microwave Sintering by Synchrotron Radiation Computed Tomography

In this study, synchrotron radiation computed tomography was applied to investigate the mechanisms of titanium powder microwave sintering in situ. On the basis of reconstructed images, we observed that the sintering described in this study differs from conventional sintering in terms of particle smoothing, rounding, and short-term growth. Contacted particles were also isolated. The kinetic curves of sintering neck growth and particle surface area were obtained and compared with those of other microwave-sintered metals to examine the interaction mechanisms between mass and microwave fields. Results show that sintering neck growth accelerated from the intermediate period; however, this finding is inconsistent with that of aluminum powder microwave sintering described in previous work. The free surface areas of the particles were also quantitatively analyzed. In addition to the eddy current loss in metal particles, other heating mechanisms, including dielectric loss, interfacial polarization effect, and local plasma-activated sintering, contributed to sintering neck growth. Thermal and non-thermal effects possibly accelerated the sintering neck growth of titanium. This study provides a useful reference of further research on interaction mechanisms between mass and microwave fields during microwave sintering.

On the self-damping nature of densification in photonic sintering of nanoparticles.

Sintering of nanoparticle inks over large area-substrates is a key enabler for scalable fabrication of patterned and continuous films, with multiple emerging applications. The high speed and ambient condition operation of photonic sintering has elicited significant interest for this purpose. In this work, we experimentally characterize the temperature evolution and densification in photonic sintering of silver nanoparticle inks, as a function of nanoparticle size. It is shown that smaller nanoparticles result in faster densification, with lower temperatures during sintering, as compared to larger nanoparticles. Further, high densification can be achieved even without nanoparticle melting. Electromagnetic Finite Element Analysis of photonic heating is coupled to an analytical sintering model, to examine the role of interparticle neck growth in photonic sintering. It is shown that photonic sintering is an inherently self-damping process, i.e., the progress of densification reduces the magnitude of subsequent photonic heating even before full density is reached. By accounting for this phenomenon, the developed coupled model better captures the experimentally observed sintering temperature and densification as compared to conventional photonic sintering models. Further, this model is used to uncover the reason behind the experimentally observed increase in densification with increasing weight ratio of smaller to larger nanoparticles.